fix: Send pre-message after successful sending of post-message (#8063)

On the receiving side, download post messages w/o known pre-message after 30s to protect from lost
messages.

This isn't easy to test in Rust though because the `smtp` sending code requires an SMTP connection,
so it's only tested (by already existing tests) that messages are queued in the right order.

Another problem is that the `smtp` sending logic doesn't try to send any messages following a
failed-to-send message until the retry limit is reached, so if there's smth wrong with a
post-message, other unrelated messages are delayed, but this problem has already existed.

src/sql/migrations.rs

@@ -2385,6 +2385,19 @@ UPDATE msgs SET state=19 WHERE state=24; -- Change OutPreparing to OutFailed.
         .await?;
     }
 
+    inc_and_check(&mut migration_version, 153)?;
+    if dbversion < migration_version {
+        sql.execute_migration(
+            "
+CREATE INDEX smtp_index_msg_id ON smtp (msg_id, id);
+ALTER TABLE available_post_msgs ADD COLUMN timestamp INTEGER DEFAULT 0 NOT NULL;
+CREATE INDEX available_post_msgs_timestamp ON available_post_msgs (timestamp);
+            ",
+            migration_version,
+        )
+        .await?;
+    }
+
     let new_version = sql
         .get_raw_config_int(VERSION_CFG)
         .await?